The present invention generally relates to magnetic disc apparatuses, and more particularly to a magnetic disc apparatus that uses both high-density and low-density magnetic discs as the storage media and that can provide favorable write and read processing for the write and read modes, respectively.
In conventional magnetic disc apparatuses that perform low-density read/write using floppy discs as the storage media, the magnetic layer is magnetized to such a deep portion that an adequate overwrite characteristic and read margin are obtainable. To achieve this, the core gap length of the write/read head is set at a maximum of 0.60 times the minimum field conversion length for the write/read signal current, and the resolution at the innermost peripheral track of the magnetic disc is set to have an absolute value of 70% or more.
In magnetic disc apparatuses that perform high-density write/read and that have been proposed to date, the core gap length for the write/read head is made smaller so that the minimum field conversion length can be made smaller. The depth of magnetization is therefore not as deep and the thickness of the magnetic layer of the magnetic disc can therefore be reduced.
Moreover, magnetic disc apparatuses having mode switching to perform both low-density write/read and high-density write/read have also been proposed to be compatible with both low-density and high-density magnetic discs. Magnetic disc apparatuses of this type have a single write/read head to perform write and read for both the low-density mode and the high-density mode.
However, there are the following problems with this type of low-density/high-density compatible magnetic disc apparatus. The example used to explain these problems is a magnetic disc apparatus which uses a magnetic disc with a diameter of 90 mm (3.5 inches) and has a high-density write/read mode in which the recording density is 2MB and a low-density write/read mode in which the recording density is 1MB.
In the 2MB mode (high-density write/read mode), there is the restriction that the core gap length of the head is 0.9 .mu.m or less (1.46 .mu.m.times.0.60=0.876=0.9 .mu.m, where 1.46 .mu.m indicates the minimum field conversion length for the 2MB mode) so that the absolute value of the resolution at the innermost peripheral track of the magnetic disc has an absolute value of 70% or more.
The average thickness of the magnetic layer in the write portion of a magnetic disc for the 2MB write/read is 1.0 .mu.m but the average thickness of the magnetic layer in the write portion of a magnetic disc for the 1MB write/read is 2.0 .mu.m which is approximately twice that of the magnetic disc for the 1MB write/read. In addition, the minimum field conversion length (min. FCL) is 1.46 .mu.m for the 2MB write and 2.91 .mu.m for the 1MB write.
Magnetic discs exclusively for the 2MB write/read have a thin (approximately 1 .mu.m) magnetic layer and so a head with the abovementioned gap length (0.9 .mu.m) can magnetize the magnetic layer to a sufficient depth during the write. However, magnetic discs exclusively for the 1MB write/read have a thick (approximately 2 .mu.m) magnetic layer and so when one of these discs is used in a low-density/high-density compatible magnetic disc apparatus having a head with the abovementioned gap length (0.9 .mu.m), it is impossible to magnetize deep into the magnetic layer which is approximately twice as thick.
FIG. 1 is a sectional diagram illustrating this status, where 1 and 2 indicate the head cores, g indicates the core gap length, 3 indicates the magnetic layer, and 4 indicates the base part of the disc. When the narrow head gap of this 2MB write/read head is used to write signals at the minimum field conversion length, the magnetic flux distribution curves of this write/read head can only magnetize down to the depth indicated by d in FIG. 1. Therefore, sufficient overwrite is possible for old data written with a conventional type of head (having a narrow gap) as shown in FIG. 1, but there is a poor overwrite characteristic for old data written with a magnetic disc apparatus exclusively for the 1MB mode (having a write/read head with a wide gap length of about 1.3 to 1.5 .mu.m).
With conventional low-density/high-density compatible magnetic disc apparatus, this write/read head has a narrow core gap length set to 0.6 to 0.9 .mu.m for use in either the low-density or high-density mode. This causes no problem for write to 2MB magnetic discs, but there is an insufficient depth of magnetization with respect to the thick magnetic layer for write with respect to 1MB magnetic discs and this creates the problem of a poor overwrite characteristic.
If the core gap length of the magnetic head is widened so that a sufficient depth of magnetization is obtained with respect to the 1MB magnetic discs, the resolution for high-density (2MB) write will drop. If this resolution drops to 60% or less, then there will be much interference between the waveforms of adjacent bits so that the peak shift indicated in FIG. 2 occurs to hinder favorable write and read processing.
FIG. 2 shows the read waveform with respect to input data that is written at a high density. In FIG. 2, (A) represents the data pattern, (B) represents the write data and (C) represents the write current. For (C), the period for which the write current is applied (indicated by "X" in the figure) corresponds to the minimum field conversion length. In FIG. 2, (D) indicates an isolated read waveform with respect to each magnetic conversion when the resolution is 100%, and (E) indicates an isolated read waveform with respect to each magnetic conversion when the resolution is 60%. FIG. 2 shows that the peak shift indicated by .DELTA.I occurs due to interference between the waveforms of adjacent bits when the resolution becomes about 60%.
Simply setting the core gap length to enable low-density write therefore means that it is impossible to avoid lowering the read margin for data written at high-density. Accomplishing these mutually incompatible objectives has therefore involved compromising the range for which suitable characteristics could be obtained.